QKD system polarization modulation method based on Sagnac ring

Abstract

The invention discloses a QKD system polarization modulation method based on a Sagnac ring. The method comprises the following steps: S1, carrying out beam splitting on an input optical pulse to obtain a first optical pulse and a second optical pulse; s2, clockwise transmitting the first optical pulse along a slow axis of a polarization maintaining optical fiber in a Saganc ring; the second optical pulse is transmitted anticlockwise along the slow axis of the polarization maintaining optical fiber in the Saganc ring; s3, enabling the first optical pulse and the second optical pulse to generate a phase difference; s4, adjusting the first optical pulse and the second optical pulse to be equal in power and orthogonal in polarization state; and S5, performing interference on the first optical pulse and the second optical pulse to complete polarization modulation. According to the invention, phase noise introduced by a transmission medium can be offset well through the Saganc ring, and extra phase compensation is not needed; and the clockwise and anticlockwise optical pulses in the Saganc ring are transmitted through the slow axis of the polarization maintaining optical fiber, so that the influence of polarization mode dispersion and the influence of the action difference of the phase modulator on the optical pulses in different polarization states are avoided, and the stability of the QKD system is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of quantum information and optical communication, and in particular to a polarization modulation method of a QKD system based on a Sagnac ring. Background Art

[0002] Quantum technology is a new track for the future industry. Quantum Key Distribution (QKD) research uses various modulation methods and is expanding from point-to-point applications and experimental systems to quantum network architecture. The existing QKD system architecture is as follows: Figure 1 shown.

[0003] There are currently a variety of polarization modulation schemes. In the polarization modulation scheme based on Mach-Zehnder interferometer, two orthogonal polarization components enter different arms of the interferometer, and then a phase shift is applied between them to achieve polarization encoding. Although this method is easy to implement, the Mach-Zehnder interferometer is inherently unstable because it is sensitive to external environmental interference.

[0004] Some polarization modulation schemes are based on lithium niobate phase modulators, whose polarization-maintaining fiber input is aligned with 45°, but since the polarization mode dispersion is located between the two orthogonal components (common lithium niobate phase modulators are about 10ps), high birefringence fiber or another phase modulator is usually required for additional compensation. At the same time, since the two orthogonal components are transmitted on different axes of the phase modulator, the stability of this scheme is not ideal.

[0005] Therefore, the stability of existing polarization modulation schemes is not good enough, and engineering development and application are difficult. Summary of the invention

[0006] In order to solve the problem that the stability of the existing polarization modulation scheme is not good enough, the present invention provides a QKD system polarization modulation method based on Sagnac loop.

[0007] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0008] A polarization modulation method of a QKD system based on a Sagnac loop, wherein the QKD system includes n pairs of transmitters and receivers, where n≥1; the method includes the following steps:

[0009] S1: Splitting an input optical pulse into a first optical pulse and a second optical pulse;

[0010] S2: Input the first optical pulse into the Saganc ring and transmit it clockwise along the slow axis of the polarization-maintaining optical fiber in the Saganc ring;

[0011] The second optical pulse is input into the Saganc ring and transmitted counterclockwise along the slow axis of the polarization-maintaining optical fiber in the Saganc ring;

[0012] S3: performing phase modulation on the first optical pulse or the second optical pulse so that a phase difference is generated between the first optical pulse and the second optical pulse;

[0013] S4: Determine whether the first optical pulse and the second optical pulse have equal powers and orthogonal polarization states;

[0014] If yes, directly execute step S5;

[0015] If not, the first optical pulse and the second optical pulse are adjusted to have equal power and orthogonal polarization states, and then step S5 is performed;

[0016] S5: Interfere the first light pulse with the second light pulse to complete polarization modulation.

[0017] In the above scheme, the phase noise introduced by the transmission medium can be well offset by the Saganc ring, and no additional phase compensation is required; and the clockwise and counterclockwise optical pulses in the Saganc ring are both transmitted through the slow axis of the polarization-maintaining optical fiber, so they will not be affected by polarization mode dispersion and the difference in the effect of the phase modulator on optical pulses in different polarization states, thereby improving the stability of polarization modulation.

[0018] Preferably, in step S1, the input light pulse is 45° linearly polarized light |S 0 >, expressed as:

[0019]

[0020] Among them, |H> represents horizontally polarized light, and |V> represents vertically polarized light.

[0021] Preferably, the optical pulse coupled to the polarization-maintaining fiber in the Saganc ring |S 1 > is represented as:

[0022]

[0023] Among them, |H> a represents counterclockwise horizontal linear polarization, |H> c It represents horizontal linear polarized light in clockwise direction, and i represents an imaginary number.

[0024] Preferably, in step S3, the first optical pulse and the second optical pulse arrive at the phase modulator at different times, thereby The first optical pulse is loaded onto one of the optical pulses to cause a phase difference between the first optical pulse and the second optical pulse.

[0025] Preferably, the phase modulated optical pulse |S2 > is represented as:

[0026]

[0027] Among them, |H> a represents counterclockwise horizontal linear polarization, |H> c It represents horizontal linear polarized light in clockwise direction, and i represents an imaginary number.

[0028] Preferably, the light pulse after the interference between the first light pulse and the second light pulse |S 3 > is represented as:

[0029]

[0030] Among them, |L> represents horizontal linear polarized light, i represents an imaginary number, represents the phase of phase modulation loading, and |V> represents vertical linear polarization light.

[0031] Preferably, the phase of the phase modulation loading The light pulse output after interference is 135° linearly polarized light |N>, expressed as:

[0032] Preferably, the phase of the phase modulation loading The light pulse output after interference is right-handed circularly polarized light |R>, expressed as:

[0033] Preferably, the phase of the phase modulation loading The light pulse output after interference is 45° linearly polarized light |P>, expressed as:

[0034] Preferably, the phase of the phase modulation loading The light pulse output after interference is left-handed circularly polarized light |L>, expressed as:

[0035] Beneficial technical effects of the present invention:

[0036] The present invention provides a polarization modulation method for a QKD system based on a Sagnac ring. The phase noise introduced by a transmission medium can be well offset by the Saganc ring without the need for additional phase compensation. Moreover, both clockwise and counterclockwise optical pulses in the Saganc ring are transmitted through the slow axis of a polarization-maintaining optical fiber, and thus will not be affected by polarization mode dispersion and the difference in the effect of a phase modulator on optical pulses in different polarization states, thereby improving the stability of polarization modulation. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 is a block diagram of the QKD system;

[0038] Figure 2 It is a flow chart of the implementation steps of the technical solution of the present invention. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments, but the scope of protection claimed in the present invention is not limited to the following specific embodiments.

[0040] Example 1

[0041] like Figure 2 As shown, a polarization modulation method of a QKD system based on a Sagnac loop, the QKD system includes n pairs of transmitters and receivers, n≥1; the method includes the following steps:

[0042] S1: Splitting an input optical pulse into a first optical pulse and a second optical pulse;

[0043] S2: Input the first optical pulse into the Saganc ring and transmit it clockwise along the slow axis of the polarization-maintaining optical fiber in the Saganc ring;

[0044] The second optical pulse is input into the Saganc ring and transmitted counterclockwise along the slow axis of the polarization-maintaining optical fiber in the Saganc ring;

[0045] S3: performing phase modulation on the first optical pulse or the second optical pulse so that a phase difference is generated between the first optical pulse and the second optical pulse;

[0046] S4: Determine whether the first optical pulse and the second optical pulse have equal powers and orthogonal polarization states;

[0047] If yes, directly execute step S5;

[0048] If not, the first optical pulse and the second optical pulse are adjusted to have equal power and orthogonal polarization states, and then step S5 is performed;

[0049] S5: Interfere the first light pulse with the second light pulse to complete polarization modulation.

[0050] In the specific implementation process, the phase noise introduced by the transmission medium can be effectively offset by the Saganc ring, and no additional phase compensation is required; and the clockwise and counterclockwise optical pulses in the Saganc ring are both transmitted through the slow axis of the polarization-maintaining optical fiber, so they will not be affected by the polarization mode dispersion and the difference in the effect of the phase modulator on optical pulses in different polarization states, thereby improving the stability of polarization modulation.

[0051] Example 2

[0052] A polarization modulation method for a QKD system based on a Sagnac loop comprises the following steps:

[0053] S1: Splitting an input optical pulse into a first optical pulse and a second optical pulse;

[0054] More specifically, in step S1, the input light pulse is 45° linearly polarized light |S 0 >, expressed as:

[0055]

[0056] Among them, |H> represents horizontally polarized light, and |V> represents vertically polarized light.

[0057] S2: Input the first optical pulse into the Saganc ring and transmit it clockwise along the slow axis of the polarization-maintaining optical fiber in the Saganc ring;

[0058] The second optical pulse is input into the Saganc ring and transmitted counterclockwise along the slow axis of the polarization-maintaining optical fiber in the Saganc ring;

[0059] More specifically, the optical pulse |S coupled to the polarization-maintaining fiber in the Saganc ring 1 > is represented as:

[0060]

[0061] Among them, |H> a represents counterclockwise horizontal linear polarization, |H> c It represents horizontal linear polarized light in clockwise direction, and i represents an imaginary number.

[0062] S3: performing phase modulation on the first optical pulse or the second optical pulse so that a phase difference is generated between the first optical pulse and the second optical pulse;

[0063] More specifically, in step S3, the first optical pulse and the second optical pulse arrive at the phase modulator at different times, thereby changing the phase The first optical pulse is loaded onto one of the optical pulses to cause a phase difference between the first optical pulse and the second optical pulse.

[0064] In the specific implementation process, the two optical pulses of the clockwise and counterclockwise channels arrive at the phase modulator at different times and have a fixed delay. Therefore, by adjusting the delay of the control electrical signal of the phase modulator, the phase can be accurately adjusted. Loaded onto one of the light pulses, so that the two light pulses have a certain phase difference.

[0065] More specifically, in this embodiment, a lithium niobate phase modulator is used to phase modulate the horizontal linear polarized light. The optical pulse|S 2 > is represented as:

[0066]

[0067] Among them, |H> a represents counterclockwise horizontal linear polarization, |H> c It represents horizontal linear polarized light in clockwise direction, and i represents an imaginary number.

[0068] S4: Determine whether the first optical pulse and the second optical pulse have equal powers and orthogonal polarization states;

[0069] If yes, directly execute step S5;

[0070] If not, the first optical pulse and the second optical pulse are adjusted to have equal power and orthogonal polarization states, and then step S5 is performed;

[0071] S5: Interfere the first light pulse with the second light pulse to complete polarization modulation.

[0072] More specifically, the light pulse after the first light pulse and the second light pulse interfere with each other |S 3 > is represented as:

[0073]

[0074] Among them, |H> represents horizontally polarized light, i represents an imaginary number, represents the phase of phase modulation loading, and |V> represents vertical linear polarization light.

[0075] More specifically, the optical pulses output after interference can be loaded with a phase modulator. To control:

[0076] When the phase modulation load is When , the light pulse output after interference is 135° linearly polarized light |N>, expressed as:

[0077] When the phase modulation load is When , the light pulse output after interference is right-handed circularly polarized light |R>, which can be expressed as:

[0078] When the phase modulation load is When , the light pulse output after interference is 45° linearly polarized light |P>, expressed as:

[0079] When the phase modulation load is When , the light pulse output after interference is left-handed circularly polarized light |L>, which can be expressed as:

[0080] In the specific implementation process, these four mutually unbiased bases meet the requirements of the BB84 protocol and can be used for quantum key distribution. If necessary, these four polarization states can be converted into the common |H>, V>, |L>, |R> through unitary changes.

[0081] According to the disclosure and teaching of the above description, those skilled in the art to which the present invention belongs may also make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments disclosed and described above, and some modifications and changes to the invention should also fall within the scope of protection of the claims of the present invention. In addition, although some specific terms are used in this specification, these terms are only for the convenience of description and do not constitute any limitation to the present invention.

Claims

1. A polarization modulation method for a QKD system based on a Sagnac loop, wherein the QKD system comprises n pairs of transmitters and receivers, where n≥1; characterized in that: The method comprises the following steps: S1: Splitting an input optical pulse into a first optical pulse and a second optical pulse; S2: Input the first optical pulse into the Saganc ring and transmit it clockwise along the slow axis of the polarization-maintaining optical fiber in the Saganc ring; The second optical pulse is input into the Saganc ring and transmitted counterclockwise along the slow axis of the polarization-maintaining optical fiber in the Saganc ring; S3: performing phase modulation on the first optical pulse or the second optical pulse so that a phase difference is generated between the first optical pulse and the second optical pulse; S4: Determine whether the first optical pulse and the second optical pulse have equal powers and orthogonal polarization states; If yes, directly execute step S5; If not, the first optical pulse and the second optical pulse are adjusted to have equal power and orthogonal polarization states, and then step S5 is performed; S5: Interfere the first light pulse with the second light pulse to complete polarization modulation.

2. The polarization modulation method of a QKD system based on a Sagnac loop according to claim 1, characterized in that: In step S1, the input light pulse is 45° linearly polarized light |S0>, expressed as: Among them, |H> represents horizontally polarized light, and |V> represents vertically polarized light.

3. The polarization modulation method of a QKD system based on a Sagnac loop according to claim 1, characterized in that: The optical pulse |S1> coupled to the polarization-maintaining fiber in the Saganc ring is expressed as: Among them, |H> a represents counterclockwise horizontal linear polarization, |H> c It represents horizontal linear polarized light in clockwise direction, and i represents an imaginary number.

4. The polarization modulation method of a QKD system based on a Sagnac loop according to claim 1, characterized in that: In step S3, the first optical pulse and the second optical pulse arrive at the phase modulator at different times, thereby changing the phase The first optical pulse is loaded onto one of the optical pulses to cause a phase difference between the first optical pulse and the second optical pulse.

5. The polarization modulation method of a QKD system based on a Sagnac loop according to claim 4, characterized in that: The phase modulated optical pulse |S2> is expressed as: Among them, |H> a represents counterclockwise horizontal linear polarization, |H> c It represents horizontal linear polarized light in clockwise direction, and i represents an imaginary number.

6. The polarization modulation method of a QKD system based on a Sagnac loop according to claim 1, characterized in that: The optical pulse |S3> after the interference of the first optical pulse and the second optical pulse is expressed as: Among them, |H> represents horizontally polarized light, i represents an imaginary number, represents the phase of phase modulation loading, and |V> represents vertical linear polarization light.

7. The polarization modulation method of a QKD system based on a Sagnac loop according to claim 6, characterized in that: Phase modulation loading phase The light pulse output after interference is 135° linearly polarized light |N>, expressed as:

8. The polarization modulation method of a QKD system based on a Sagnac loop according to claim 6, characterized in that: Phase modulation loading phase The light pulse output after interference is right-handed circularly polarized light |R>, expressed as:

9. The polarization modulation method of a QKD system based on a Sagnac loop according to claim 6, characterized in that: Phase modulation loading phase The light pulse output after interference is 45° linearly polarized light |P>, expressed as:

10. The polarization modulation method of a QKD system based on a Sagnac loop according to claim 6, characterized in that: Phase modulation loading phase The light pulse output after interference is left-handed circularly polarized light |L>, expressed as:

Images